Photodiodes and solar cells are often characterized by quantum efficiency (QE) and responsivity to measure the optical-to-electrical conversion efficiency of the device. Quantum Efficiency is expressed in units of outgoing electrons per incident photon, while Responsivity is expressed in units of outgoing current per incident Watt (A/W). Further, spectral response (SR) is a measure of the device conversion efficiency as a function of incident photon wavelength or optical frequency. Spectral response can be expressed as either QE or responsivity with respect to wavelength by a simple conversion of units.
Historically, apparatuses used to measure QE have used a conventional broadband lightsource such as Quartz Tungsten Halogen, Xenon Arc, or Metal Halide, where the light is spectrally resolved with either a wavelength scanning monochromator or a set of bandpass filters. For example, FIG. 1 shows a conventional QE measurement system 100 that includes a lamp 102, a monochromator 104, an order sorting filter 107, an optical chopper 108, a lens system 110, a quasi-monochromatic light 112, a device under test (DUT) 114, and a current monitor 116 (typically a lock-in amplifier or other synchronous detection circuit).
QE measurements are made sequentially as the apparatus is mechanically stepped through a series of predetermined wavelengths by adjusting either the monochromator 104 diffraction grating 106 angle or placing individual bandpass filters into the beam of broadband light. The resultant light 112 (often called monochromatic, but typically exhibiting a bandwidth in the range of 5-20 nm) is directed at the DUT 114, and the current output of the DUT 114 is recorded and normalized to the incident light 112 intensity.
Monochromator-based systems as described in FIG. 1 offer adjustable wavelength resolution and essentially continuous coverage over the range of interest. For solar cells, the range of interest can include wavelengths from the shortest solar emissions around 300 nm to the wavelength corresponding to the smallest bandgap present in the active region of the device, for example approximately 1100 nm for Silicon, and longer for some other materials. However, when scanning such a large wavelength range, monochromators require the use of order-sorting filters to prevent higher order diffraction (λ/2, λ/3 . . . ) from reaching the output slit, and also leak a measurable amount of broadband stray light that can not readily be removed by filters.
Interference filter-based systems offer good stray light rejection, but use multi-layer dielectric films that eventually degrade due to direct exposure to the high intensity, and UV-containing, broadband lightsource. Filters must be regularly checked and replaced to preserve data integrity. Filter designs with wide rejection bands (required for use with a broadband light source) tend to be both inefficient (with peak transmission <50%) and expensive. Such filters tend not to be tunable (via tilting the angle of incidence with respect to the filter) over a wide wavelength range: typical tuning ranges are ˜5% of the nominal wavelength. Traditional filter designs with wide rejection bands suffer significant increase in losses as they are tuned via tilting.
Both monochromators and interference filters require a collimated incident beam in order to function optimally, with an increasingly tight collimation requirement as the passband width is decreased. Because conventional sources emit light with a poor etendue, a significant throughput penalty must be suffered when integrating such sources with a monochromator. This throughput penalty is in addition to the inherent losses associated with selecting a small portion of the lamp spectrum, and with the internal inefficiencies of the monochromator itself. Hence, conventional sources cannot deliver spectrally selected light efficiently to a QE measuring system. This is the origin of the very high power consumption that is common to existing systems. Additionally, the unstable light output (both short term and long term) of conventional light sources necessitates frequent calibrations and bulb replacement.
In both monochromator and interference filter systems, a basic limitation is the speed with which the system can scan through a set of wavelengths. Mechanical motion must occur to step from one wavelength to the next, and this limits the practical throughput of the system. Due to the small fraction of total light striking the sample at any time, scans may take ˜10-30 seconds per wavelength, and involve mechanical movement of the filters and the use of a beam chopper and lockin (synchronous) amplifier that detects the electrical response. Additionally, variable neutral density filters or other mechanical apertures may be needed to control light intensity. With this measurement overhead, full spectra often take 5-10 minutes to cover the necessary wavelength range with sufficient resolution. This required mechanical motion also increases the cost and complexity of the instrument and makes it less suitable for a manufacturing environment.
The long measurement time of conventional QE systems prevents them from effectively being used in a mapping mode (10's to 1000's of data points per sample) to study localized or spatially varying effects. Also, the poor etendue of the conventional source sets a minimum practical measurement area, such that any reduction in measurement area below this value creates a further tradeoff of spatial resolution and measurement speed. Depending on the structural details and target application of the Device Under Test (DUT), it may be desirable to resolve millimeter spatial scales. To accomplish this a high-speed QE technique is required.
In one attempt to improve the art, an optical spectroscopy method was used to determine the quantum efficiency (QE) of a solar or PV cell, and the determination is performed in less than a minute. The system included a light source that produced multiple wavelengths concurrently and later independently processed them to determine QE at each of the wavelengths. The light source included an array of light emitting diodes (LEDs), rather than the standard white light source provided by a single halogen light bulb, for simulating the full spectrum of sunlight. A power source was used to individually drive each LED at its own unique operating frequency. The power source was a modulated power supply using a sinusoidal-wave modulated or a square-wave modulated power supply. The light source can also operate in the constantly ON mode too, where all or some of the LEDs are constantly on.
During the QE measurement of the solar cell, all of the LEDs in the array are driven “concurrently” to illuminate the solar cell. The AC current generated in the solar cell from the light transmitted by the light source was signal processed such that it was amplified and converted into a digital voltage signal (e.g., a signal made up of the individual signals corresponding to the unique operating frequency of each LED in the LED array). The use of sinusoidal power supplies expedited the use of a Fast Fourier Transform (FFT) module or algorithm run by a computer to determine the power spectrum of the current in the solar cell as a function of drive frequency from each LED light, where the voltage waveform associated with each operating frequency was converted into an amplitude associated with each drive frequency or LED. A reference cell was used to calibrate the amplitude of the FFT signals such that QE measurement module was run by the computer processor to calculate a QE value for each operating frequency or LED or light wavelength by applying a conversion factor obtained through use of the reference cell to each amplitude to generate and display a QE curve. Though this resulted in a reduction in the determination time, the resulting QE measurement included significant margins of error, due to poor spectral control of the LED light source and because light reflected, scattered or transmitted by the DUT was not measured and accounted for. Thus, the resulting QE determination provided no information relating to an internal QE value, which provides a most accurate measurement of the integrity of the DUT.
Accordingly, there is a need to develop a system for accurately and quickly measuring both a general QE and an internal QE that is low cost and easy to implement.